Patent GB-680321-A (GB)
Title: Improvements in or relating to control systems for engines driving propellers
IPC: F02C9/28
Filed: 1948-03-15
Abstract: 680,321. Gas turbine plant. BENDIX AVIATION CORPORATION. March 15, 1948 [March 15,1947], No. 7788/48. Class 110 (iii) [Also in Groups XXIX, XXXIII and XXXV] A control system for a gas turbine engine driving a variable pitch propeller has the valve controlling the fuel supply to the engine and the actuating element for varying the pitch of the propeller interconnected by means controlled independently of engine speed by a manually-operable element, and an overspeed device acts on the interconnecting means to modify or correct the relationship between propeller pitch and fuel feed rate. In an aircraft power plant, actuation by the pilot of a control quadrant 61, Fig. 2. acts through a resilient link 74 and a lever 63 to allow a spring-loaded fuel valve 60 to open, and, at the same time, the lever actuates the pitch-changing mechanism of the propeller 26<SP>1</SP> through a further lever 89, a link 90, and a lever 29. The latter carries one contact of a switch, and in moving in either direction, it completes the circuit of a reversible electric motor 109, Fig. 3, which, through a worm 109' and a gear wheel 106<SP>1</SP>, moves a sleeve gear 106 longitudinally, the latter being connected to the pitch-changing mechanism in the propeller hub. The contacts 108<SP>1</SP>, 108" which co-operate with the contact on lever 29 are carried by gear wheel 106' whereby, as the latter is rotated, it tends to re-centre lever 29 and so bring the pitch-changing mechanism to rest. An engine-speed control of the pitch is given by the normal engine-driven governor 94, Fig. 2, which actuates a pilot valve 98 controlling a servopiston 92<SP>1</SP> acting on lever 89. The speed of the aircraft also exercises a control of the propeller pitch through mechanism including a pitot tube with an impact head 112 and a static head 115. The pressures developed by the pitot tube are applied differentially to a diaphragm 117 which actuates a valve 125 through a cam 119 and a lever 123, the cam being of parabolic or logarithmic contour to compensate for the square root relationship between air speed and the pressure differential acting on the diaphragm. Valve 125 is the pilot valve for a servo-piston 126 acting on lever 89. Lever 123 is fulcrummed on the stem of servo-piston 126 so that the latter " follows-up " the movement of its pilot valve. A correction for the density of the atmosphere is provided by a bellows 133 which adjusts a valve controlled orifice in the passage from the static head 115 to the appropriate side of diaphragm 117, and this orifice is balanced against a bleed 137 bye-passing the impact pressure across the diaphragm. When valve 60 is closed, an idling range is provided by a further valve 55 in parallel with valve 60 and actuated by a separate control quadrant 56. When the latter reaches its upper limiting position, a spring-pressed locking bolt 70 enters a recess therein and, at the same time, disengages itself from a recess in quadrant 61 to free the latter so that valve 60 can be opened. The amount by which valve 60 can be opened is limited by a stop 73. A spring abutment 68 allows quadrant 56 to be adjusted below its normal minimum position for starting, &c. Fuel is delivered by a pump 34 past a balanced pressure regulating valve 35, past a valve 46 controlling the pressure differential across the fuel metering valves 55, 60, past the latter valves, and past a cut-off valve 80 to the burners of the gas turbine engine. Valve 35 is under the control of a spring-loaded diaphragm 39 subject to the differential of the pressures of the fuel at the pump outlet and on the downstream side of the metering valves. A bleed 43 allows for the escape of air. Valve 46 is actuated by a spring- pressed diaphragm 45 subject to the differential of the pressures across the metering orifices, and the pressure at the downstream side of the orifices is applied to, the diaphragm through a calibrated restriction 79. A valve 76 controlling a bleed between the two sides of diaphragm 45 is actuated by a bellows 75 subject to ram pressure and temperature to give a correction according to the air density. Should an over-temperature develop in the exhaust, fuel valve 60 is moved towards the closed position by a lever 185, the latter being controlled from a thermal pick-up element 175, Fig. 6, which is in contact with the hot gases of combustion. The element 175 includes a resistance unit, such as a carbon pile, which unbalances a bridge 176 and so actuates a reversible motor 178 connected to lever 185 through gearing 181 and a link 182. The engine drives the propeller through a sun wheel carried on the engine shaft which engages a planet carrier connected to the propeller. An outer sun wheel 145, Fig 5, mounted on ball bear ings and engaged with the planet carrier moves in response to increasing torque in the drive and opens a valve 155 to allow pressure fluid to act on a diaphragm 159 which moves a pilot valve 166 controlling servo-piston 166. The latter acts through a link 167 on to a lever 168, Fig. 2. which moves the fuel valve 60 towards the closed position and so provides an overtorque control. The fluid pressure admitted past valve 155 also acts on a diaphragm 149 to tend to return the sun wheel 145. In a modification, lever 29 carries a floating lever to the two ends of which are connected, respectively, the pilot's control quadrant 61 and the overtorque servo-piston. Movement of the quadrant 61 also acts to vary the spring loading of the overtorque control diaphragm 159 and a correction for air density is imparted to this movement, this latter correction being applied by a servo-motor controlled by a capsule sensitive to ram pressure and air temperature. The fuel valve is also connected to the pilot's control quadrant but the engine governor servo-motor acts on the fuel valve. A further modification is described in which the controls mentioned in the modification above are re-arranged. Reference has been directed by the Comptroller to Specification 634,417.